In known radio systems, the digital information to be transmitted is channel-coded in a transmitter to prevent detrimental effects caused by the noise on the radio path. In addition, in CDMA (Code Division Multiple Access) radio systems, multiple access interference, MAI, which is caused by the non-orthogonality of the users' spreading codes, deteriorates the performance of the receiver and thus the capacity of the system.
The channel codes can in principle be divided into block codes and convolutional codes. Both coding methods can be used simultaneously as well. A typical code rate of convolutional coding, i.e. the ratio of the number of data bits of the user to the coded data bits of the channel, is for example ½ or ⅓. The constraint length of the channel coding expresses how many successive data bits affect the code word resulting from the channel coding of each data bit.
The channel decoder of the receiver utilizes the redundancy of data bits, whereby coding gain is achieved. If the noise interfering with each bit is assumed to be uncorrelated, the total effect of the added redundancy is that the desired signal is amplified at the same time as the effect of the noise averages out. In this way, the reliability of the bit decisions made by the receiver is increased, which improves the performance of the radio link, for example the bit error ratio is lowered.
Generally, it can be said that channel coding gives protection against the effect of additive white Gaussian noise, AWGN. Interleaving is frequently used in addition to channel coding. In interleaving, the successive bits are mixed with each other over a longer time interval in such a way that a momentary fade on the radio path would not be sufficient to make the radio make the radio signal of that time interval unrecognizable but that the errors affecting it could still be cancelled by means of the decoding of the channel coding.
Space-time coding has been presented to improve the performance of the channel coding on fading channels. This new way of coding refers to channel coding in which also spatial diversity is utilized. One way to implement the spatial diversity is to use transmitter antennas that are sufficiently far away, for example at 10 to 20 wavelengths, from each other. Another way to implement the diversity is to use polarization diversity, in which a signal is transmitted from antennas using different polarization levels.
One of the simplest ways to implement the time-space coding is to code the data with a channel code, for example with a ½ convolutional code and then to convert the serial data flow into parallel. The conversion can be made in such a way, for instance, that two successive symbols of a code word are transmitted in a parallel manner. Each of the parallel data flows is then transmitted via a transmitter antenna of its own. More advanced codes, for example trellis codes, can also be used.
In other words, channel coding utilizes redundancy in the time domain to overcome the problems on the AWGN channel. Channel coding is not sufficient on its own on fading channels, because the coherence time of the channel is usually much longer than the duration of a symbol. This is alleviated by interleaving, but it causes significant delays in decoding, particularly in environments where the subscriber terminal moves slowly, for instance indoors. The problem of the delays in the decoding makes the designing of receivers operating at a high transmission speed particularly indoors more difficult.
In an uplink, in particular, MUD (multi-user detection) receivers utilizing interference cancellation can be used to fight the effects of the multiple access interference. This type of receiver receives signals from several users, and then the multiple access interference caused by the other users is estimated and cancelled from the received signal so as to receive the signal of the desired user.
Incorporated herein by reference is the article by Muszynski, Peter: Interference Rejection Rake-Combining For WCDMA in Proceedings of the First International Symposium on Wireless Personal Multimedia Communications (WPMC) Nov. 4-6, 1998 Yokosuka Japan, which describes an IRC (Interference Rejection Combining) receiver which contributes to eliminating the effect of interference and noise from the received signal.
Incorporated herein by reference is also the article by Bottomley, Gregory E and Jamal, Karim: Adaptive Arrays and MLSE Equalization in Proceedings of the 45th IEEE Vehicular Technology Conference (VTC) 1995, which describes the performance of MLSE equalization (Maximum Likelihood Sequence Estimation Equalization) of a receiver utilizing receiver antenna diversity so as to reduce the interference of the received signal.
The cancellation of the multiple access interference is based on tentative decisions of the other users' data symbols, and therefore it can be repeated several times, so that at each time, the new decisions are used as new tentative decisions. The reliability of the decisions is thus improved iteratively. Typically, for example two interference cancellation stages are used.
Since interleaving is used in connection with channel coding, the tentative decisions are “raw” decisions, in other words they are not based on decisions made after the decoding of the channel coding. The decoding is performed prior to the final decisions. If the channel decoding were performed prior to the estimation and cancellation of the multiple access interference, the received signal should be buffered over the length of the interleaving depth. This causes two problems: a long decision delay and large memory buffers to collect the interleaved data.